Leakage from fluid distribution systems, such as domestic water systems and compressed-air systems, is common and frequently goes undetected. Toilet leaks are often silent and are detected only when a water bill is excessively high. Leakage from toilets and from faucets in apartment buildings is difficult for the manager to detect and thus to deal with. Leakage from plant compressed-air systems frequently goes unnoticed for years, and it would be difficult for the plant manager to locate the leakage, particularly if valving to isolate portions of the system were not available.
In most fluid distribution systems of the types described, fluid drawn for useful purposes is drawn intermittently, while leakage occurs continuously while the system is in operation. A device described by Meyer (U.S. Pat. No. 4,518,955) detects leakage by determining whether or not flow is above two preset values, and indicating leakage if the flow exceeds either of these values for a corresponding period of time. The device involves a mechanical flow sensor, capable of distinguishing specific levels of flow.
A device described by Abe (U.S. Pat. No. 4,437,336) determines when the flow rate is below a predetermined value. It integrates the flow during such periods, taking it to be leakage flow. This approach has the obvious disadvantage of not distinguishing high rates of leakage from low rates of useful flow.
A number of thermal fluid flow sensing devices have been developed. They apply heat to a fluid, usually at a fixed rate, and take a rise in the temperature at a point in the fluid, usually relative to the temperature at another point in the fluid, as an indication that flow has stopped. They differ from thermal flowmeters in that they are designed for maximum sensitivity to small rates of flow, rather than for predictable response to a range of flow rates.
One such thermal fluid flow sensing device is described by Seeley (U.S. Pat. No. 3,438,254). It consists of a heating coil for warming the pipe through which the fluid flows, temperature-sensing coils wound around the pipe for sensing its temperature at specific points, and, optionally, a check valve in the pipe to block convection currents and to improve the sensitivity of the device. The preferred embodiment is intended for monitoring the flow of fluid from an oil well. The device, as described, could not be applied to an existing pipe in place and in service.
A second thermal fluid flow sensing device, again a device for detecting flow at oil wells, is described by Densmore (U.S. Pat. No. 3,570,310). It consists of a heating element and a temperature-sensitive switch, both affixed to the outside of a pipe. If there is no flow, the pipe is warmed sufficiently by the heater to activate the switch. As described, it requires having its components welded to the pipe in order to assure adequate thermal contact. This device would have to heat the pipe well above the highest expected room temperature before a stoppage of flow could be reliably determined. Consequently, its response would be slow and its heat requirement would be large.
An externally-mounted flowmeter is described by Tobin in an article entitled "Development of a Clamp-On Flowmeter" in the Proceedings of the 18th International ISA Symposium, vol. 18, May 1972 pp. 338-340. This device measures flow by determining the amount of heat required to maintain a fixed temperature difference between two sensors. It is described as being applied to one-quarter-inch hydraulic lines.
It will therefore be seen that the problem of detecting and indicating leakage from service water systems has been addressed, but only by means which require disruption of the water system and the use of skilled labor for installation. The devices also do not make good use of the history of the fluid flow rate in estimating the likely existence of leakage and the probable magnitude of such leakage. It will also be seen that the use of thermal fluid flow detectors is well known, though applications to existing piping systems without system disruption are rare. In most cases, applications are to small-diameter pipes, in which circulating flow caused by natural convection (with no net flow along the pipe) is less significant than it is in larger pipes. Leak detectors for domestic water distribution systems have not employed the technology of thermal fluid flow detection.